This invention relates to a length measuring apparatus of the type having a measuring grid scale, a plurality of reference marks positioned in association with the measuring grid scale and a scanning unit for sensing the grid scale and reference marks and generating an output signal in response thereto.
An apparatus of this type is used for example in conjunction with a tool and workpiece where it is required to measure the relative or absolute position or change of position of one with respect to the other. In such a usage, the scanning unit is typically connected to the tool and the grid scale is connected to the workpiece.
An example of this type of apparatus is described in U.S. Pat. No. 4.491.928 (corresponding to German patent DE 30 39 483 C2). In that patent, a position measuring apparatus is described having coded markings (eg. C.sub.1, C.sub.2, C.sub.3, C.sub.4 ...) allocated to the individual reference marks (N). By input of the corresponding code into a computer or comparator (R). any arbitrary reference mark (N) can be selected and located and the scanning unit can be positioned over the correspondingly-coded reference mark. Moreover, the coded marking may also include information to enable the determination of the absolute position of the reference mark (N) with respect to a reference point.
If the absolute position of the selected reference mark is not needed for a particular usage, the selection of individual reference marks can be made by means of optical or magnetic switching elements. An example of such an incremental length position measuring apparatus that provides for the determination of the relative position of two objects by means of such switching elements (e.g. magnet 9) is described in U.S. Pat. No. 4,101,764 (corresponding to German patent DE 25 40 412-C3).
Still another position measuring apparatus is described in U.S. Pat. No. 3,982,106 (corresponding to German patent DE 24 16 212-C3). In this patent, the reference marks are uniquely identifiable (or coded) by their spacings apart. Thus, by an evaluation of these spacings, the absolute position of any of the reference marks--with respect to a reference point of the apparatus--can be determined.
Users of such position measuring devices such as machine tool manufacturers, have different requirements especially relating to the various dimensions and ranges of dimensions of the tools and workpieces for which an apparatus of this type may be used. Accordingly, in order to meet these various requirements, manufacturers of position measuring devices have had to provide measuring scales of various dimensions to accommodate these needs. For the manufacturer, this requires the production and stocking of various sizes and types of position measuring components adapted for the specific and various needs of the different users of such devices. This is undesirable due to the relatively high cost of such production and storage.
Accordingly, it is an object of the present invention to provide a position measuring apparatus that is adaptable to the requirements of various users yet minimizes the production of a multiplicity of components.
It is a further object of the present invention to provide a position measuring apparatus that can accommodate various users by a standardization of specialized and expensive components.